Nasty worm poses as MS security update

Swen: old trick, new packaging

Updated Windows users were yesterday warned of the appearance of a worm that poses as a security update from Microsoft but actually causes all manner of mischief on infected PCs.

Swen-A (AKA Gibe-F) is a mass-mailing worm that also attempts to spread through file-sharing networks, such as KaZaA and IRC, and over local area network shares. The worm attempts to de-activate antivirus and personal firewall programs running on an infected computer.

AV vendors warn that the worm is spreading rapidly and that disinfection is difficult. As usual this is a Windows-only menace - Linux, Mac, OS/2 and Unix users are immune.

Managed services firm MessageLabs reports today that it has blocked copies of Swen-A 35,400 times since first intercepting it on September 14. Initial copies all originated from Slovakia, and some later copies originated from the Netherlands, the company reports, adding that MessageLabs' subscribers in the US, UK and the Netherlands have been most heavily targeted by the worm.

Swen-A uses a well known vulnerability in Internet Explorer to execute directly from e-mail. Windows users can also catch the pox by executing an infected email attachment.

Finnish AV firm F-Secure compares the worm to Gibe, and believes it is likely that the same author wrote both worms.

Swen-A (like Gibe and numerous other viruses before it) purports to be a security alert from Microsoft. This time around infectious messages come with a well-presented HTML message complete with graphics that are more likely to trip up the unwary.

The worm can also impersonate mail delivery failure notices, attaching itself as a randomly named executable.

Swen-A attempts to spread by emailing itself using its own SMTP client to addresses extracted from various sources on the victim's drives (e.g. MBX and DBX files). Periodically the worm presents users with a fake MAPI Exception error, prompting them to enter the details of their email account (name, user name, servers).

Sneaky.

Swen-A also makes modifications which make it hard to run Reg Edit, along with other changes to infected PCs explained in advisories from F-Secure and Symantec.

Windows users are advised to update the virus signature files on their AV scanners to defend themselves against the worm, which is all very well but the reason the virus got a hold in the first place is probably because of the shortcomings of the scanner model. ®